This invention relates to a fill level indicator for use in a liquefied-petroleum-gas tank. The device is particularly suitable for indicating the fill level of an automobile fuel tank.
In the case of a liquefied-petroleum-gas tank it is important to precisely monitor fill-level changes. In fact, because of fuel expansion, safety considerations dictate that the tank not be filled more than 80% of its total capacity. At the low level a tank must also provide information that allows for instance a switchover to gasoline for fueling the engine.
The gauges that have been employed to date are based on a mechanical float system which has numerous drawbacks including in particular imprecise measurements due to mechanical slack, the risk of malfunction of the mechanical elements due to seizing or wear as they age, random measurement variations due to vehicle movements, the difficulty of compartmentalizing a liquefied gas tank for limiting the movement of the fluid as in the case of a gasoline tank, their bulk in the tank, and the lack of safety.
There exist gauges which incorporate optical detection devices whose operation is based on measuring the difference in the refractive index between liquid and gaseous substances. The U.S. Pat. No. 4,286,464 describes such a fill-level indicator in a tank and especially in an oil reservoir. The fill-level indicator described in that document incorporates an array of vertically staggered optical detectors. Each detector includes a light source such as a gallium and arsenic P-N LED, a receiver such as a flat P-N-P silicon phototransistor, as well as light-beam transmitting elements. These elements transmit the beam from the light source to the receiver when the detector is out of the liquid while the beam is refracted when the detector is immersed in the liquid. Electronic circuitry permits the transmission of the signals received by the detectors to an oil-level display gauge.
It is the objective of this invention to provide a fill-level indicator for liquefied petroleum gas tanks which indicator is compact, has no moving parts, is highly reliable and can be coupled to additional safety provisions such as a fill lock which is enabled when a certain level is reached or when the engine of the vehicle is running.
The device is designed to employ optical detectors whose detection principle is based on the difference in the refractive index between liquid and gaseous substances.
Accordingly, the indicator presented incorporates the following:
An array of optical detectors on a mount, vertically spaced apart from one another inside the tank and distributed over the height of the tank, with each detector encompassing a light source and a receiver, and
Means which power the light sources of the various detectors, process the information received from the different detectors and transmit it to a liquefied-gas fill-level display gauge.
According to the invention, the mount and the detectors on it are enclosed in a synthetic resin that is highly transparent to the light beams emitted by the light sources while the surface of the resin facing the detectors is such that a light beam emitted by the corresponding light source is reflected toward the associated receiver.
This design concept permits the use of optical detectors in a liquefied petroleum gas (LPG) tank in which the ambient conditions are particularly severe. With the resin it is possible to seal the sensor assembly into one solid, single block, providing excellent electrical insulation of these components from the LPG.
The level of the gas inside the tank is measured by processing the signals that have reached the different receivers, based on the fact that each of the receivers positioned in the gaseous phase receives a light beam emitted by the corresponding light source, whereas the other receivers, i.e. those immersed in liquid gas, do not receive such signals.
In one design implementation of this device, each light source consists of a diode which emits a light beam in the visible or infrared wavelength range and each receiver is constituted of a photoelectric cell or a photothyristor.
The resin used may for instance be an epoxy, given that it has a refractive index close to that of the LPG in the liquid phase.
When a detector is positioned in the gaseous phase, the emitted light beam is reflected toward the corresponding receiver since the index of the gas, at close to 1, is optically well below the index of the resin.
When the detector is positioned in the liquid phase, the beam emitted by the light source is essentially diffused in the liquid since the refractive indices of the liquid and of the resin are similar at about 1.3 to 1.4. A small part of the light beam may still be reflected toward the receiver, but the sensitivity of the latter is not such as to register light of this weak a magnitude.
The section of the resin layer covering the detectors is not contour-matched since it is important that the light path of the beam reflected by the inner wall surface of the resin be directed from the light source toward the detector. However, it is desirable to place the light source and the receiver of a given detector quite close together and to have the surface of the resin in front of the detector assembly, composed of light source and sensor, extend parallel to the detector and its mount.
In a design variation, the mount is located in a casing that serves as an outer enclosure for the resin and is highly transparent to the beams emitted by the light sources. This casing may consist for instance of polycarbonate. The refractive index of the material constituting the casing must be as close as possible to the index of the resin and of the LPG in the liquid phase.
The casing is preferably U-shaped. Each leg of the U is provided, for instance on its inner surface, with a longitudinal groove and the two grooves serve to accept the detector mount in such fashion that it extends parallel to the base of the casing. The detector mount is thus perfectly aligned in the casing, providing good parallelism between the base plane of the casing and the plane of the detector mount. Of course, surfaces other than these planar surfaces can be utilized for the base and the mount, but the planar configuration offers the advantage of being the easiest to implement.
For fitting the probe in the tank containing the LPG, the mount, the resin and possibly the casing are enclosed in a retaining head that is attached to the tank. This retaining head may for instance be a metal head equipped with an annular flange and bolted to the tank. This allows the heed to be mounted in the location usually occupied by a traditional float-type mechanical gauge, fastened to the tank with four screws.
To ensure proper electrical connection between the detectors of the probe and the outside of the tank, an insulated wire conduit is suitably installed in the retaining head.
According to one advantageous embodiment of the indicator per this invention, the mount supporting the detectors consists of a printed circuit board.
In a preferred design implementation the means for supplying electric power to the light sources and for processing the signals include a microprocessor or microcontroller.
Since the fill-level indicator only provides discrete i.e. discontinuous measurements, it will be desirable to prevent the needle of the gauge from dropping upon every change of the state of a detector. This is accomplished in that the signal processing means perform a smoothing function on the value of the gauge shift between the corresponding measurements of two neighboring detectors, simulating intermediate measurements between two actual measuring points by the interpolation of a mean gas consumption value during an average time period.
To avoid registering every interference-induced change in the state of the detectors which does not reflect the actual level of the liquid, caused by the splashing of droplets, a wave motion due to movements of the vehicle or a leaning of the vehicle, the signal processing elements include a change-of-state filtering provision for the detectors which establishes a time period during which no variation in the detection is to be registered. This time period can be relatively short, with a duration on the order of a few seconds, depending on the desired sensitity level.
As an advantageous feature of this invention when applied to an automobile tank, the signal processing means are connected at one end to the electric distribution panel of the vehicle engine and at the other end to a solenoid valve installed on the tank filler inlet, enabling the solenoid valve to open up only when the engine is stopped and when the fluid is below a specific level, the maximum fill level being 80% of the tank capacity which corresponds to the position of the uppermost detector.
The device per this invention thus incorporates important safety functions. It should be noted that in conventional systems the overfill prevention is implemented by mechanical means employing a float, which again has the same shortcomings as those mentioned above in connection with the gauge.
To ensure highly safe operation of the device, the signal processing means include for instance a test function for all optical detectors and for the various electronic components which may be subject to possible malfunction.
For safety reasons, if the maximum-fill detector were to fail, its functions are automatically transferred to the next lower detector, or the filler solenoid valve remains closed.
The signal processing means which include a microcontroller or a microprocessor may also be designed in a way as to reduce electric power consumption when idle, i.e. when the vehicle is stopped, or to conserve the power for retaining control of the solenoid valve in the fill position. It is possible to provide these signal processing elements with an interface to the electronic fuel injection system for the purpose of enhanced performance and safety of the vehicle.